Modifications among the vertebrate classes. Birds
Bird circulatory systems have many similarities to
those of reptiles, from which they evolved.The changes
that have occurred are more of degree than of kind.
The heart is completely divided into right and left
sides. The sinus venosus is incorporated into the right
auricle and becomes the sinoauricular node. It is from
this point that the heartbeat is initiated. There is
noconus, and only two vessels leave the divided ventricle.
These are the pulmonary artery from the right side and
the systemic arch from the left. The systemic arch is
asymmetrical—the main difference in this area between
birds and lizards. Only the right part of the systemic
archis present, the left being suppressed. The arterial
arches are no longer bilaterally symmetrical. Another
difference between birds and lizards is found in the
venous circulation: the renal portal system is reduced
in birds.
